Electrical converters with multiple stacked converter cells, such as modular multi-level converters, usually require auxiliary power for the individual converter cells to supply control electronics, signal processing, sensors and gate drivers. This auxiliary power is usually drawn locally from a main cell capacitor of the respective converter cell, which may simplify the system design significantly. Otherwise, each converter cell has to be supplied with auxiliary power from an additional DC/DC converter, which has to provide full voltage insulation, which may be up to several 100 kV in case of HVDC (high voltage DC) applications. This would result in a significant increase of complexity, insulation issues and cost.
Start-up of such converters may be performed in a way that the cell capacitors are loaded in a first step. At this stage, the converter may work as a simple passive diode bridge, and the inrushing current may have to be limited by resistors which are disconnected after a DC link is charged. After this first step, the cell capacitors may be charged sufficiently in order to start up the auxiliary power supply of the converter cell, thus supplying control electronics, sensors and gate drivers. In the following second step, the control electronics may start generating gate signals and may activate the semiconductor switches of the converter cell in order to boost the DC link in a controlled way to the DC link voltage required for nominal operation. After starting up the auxiliaries, it is also possible to monitor the cell states, for example with sensors for voltage, current, temperature, etc.
However, the described procedure may not work when the semiconductor switches are normally-on switches like SiC-JFETs (junction gate field-effect transistors). In the case that such semiconductor switches do not receive a gate voltage signal, they are in on-state and short the DC link. Therefore, the cell capacitors of the individual converter cells have to be pre-charged or a mechanism to power the gate driver has to be provided before connecting to the grid in order to allow the normally-on switches to be in a controlled off-state for the short time during the first stage of starting up the converter.
Furthermore, at commissioning of a converter, it would be of advantage to check if all converter cells operate as planned, for example that all signals are working correctly, pulse width modulation signals are available, no signal connection is broken, all communication interfaces are operational, etc., before connecting the converter to an electrical grid and starting the converter up. This may be only possible if all converter cells can be energized externally, not directly from the electrical grid, which may be a high voltage grid providing more than 100 kV. In such cases it may be sufficient to provide a relatively small voltage (such as 24 V per cell) for starting up the converter cells.
For pre-charging converter cells, there are several technical solutions.
Batteries may be employed during the converter start up. This, however, requires a large number of batteries, for example at least one battery for each cell, and may result in a labor-intensive maintenance effort.
Another possibility is inductive power transfer. However, inductive power transfer may require a major redesign of the converter, because mechanically complex secondary coils may have to be integrated in each converter cell. Also, the converter cells may have to provide accessible space for placing primary coils. Furthermore, the devices for inductive power transfer may have to be designed for very high voltage. Placing them next to the converter cells may require a redesign of the insulation concept of the converter.
Also, electrical connections with switches and/or cabling, which has to be removed out of the insulation spaces before starting up, may be used to power up each converter cell. However, this usually results in a high maintenance effort, a possible redesign of the converter and/or the hall the converter is arranged, and may be labor-intensive, in particular, when there is a large number of converter cells.
WO 2016 108 552 A1 shows a power control apparatus for a submodule of a modular multi-level converter. The auxiliary power of the converter cells is provided by loading of the cell capacitor during the first step of the start-up of the converter.